1. Field
The present application relates generally to wireless communications, and more specifically to systems and methods for disambiguating the timing of uplink transmissions.
2. Background
The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) represents a major advance in cellular technology and is the next step forward in cellular 3G services as a natural evolution of Global System for Mobile communications (GSM) and Universal Mobile Telecommunications System (UMTS). LTE provides for an uplink speed of up to 50 megabits per second (Mbps) and a downlink speed of up to 100 Mbps and brings many technical benefits to cellular networks. LTE is designed to meet carrier needs for high-speed data and media transport as well as high-capacity voice support. Bandwidth is scalable from 1.25 MHz to 20 MHz. This suits the needs of different network operators that have different bandwidth allocations, and also allows operators to provide different services based on spectrum. LTE is also expected to improve spectral efficiency in 3G networks, allowing carriers to provide more data and voice services over a given bandwidth. LTE encompasses high-speed data, multimedia unicast and multimedia broadcast services.
The LTE physical layer (PHY) is a highly efficient means of conveying both data and control information between an evolved NodeB (eNB) and mobile entities (MEs), such as, for example, access terminals (ATs) or user equipment (UE). The LTE PHY employs some advanced technologies that are new to cellular applications. These include Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) data transmission. In addition, the LTE PHY uses Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink (DL) and Single-Carrier Frequency Division Multiple Access (SC-FDMA) on the uplink (UL). OFDMA allows data to be directed to or from multiple users on a subcarrier-by-subcarrier basis for a specified number of symbol periods.
In certain systems that lack an external time reference (e.g., 3GPP LTE), the periodicity of the system frame number (SFN), which provides the longest meaningful range of a time reference operable in the system, may be on the order of ten seconds. As a result, for certain purposes, such as, for example, position measurements or events that may be reported by a ME to a wireless network well after being logged by the ME, the wraparound time may be too short to give accurate timing information. Moreover, even with a longer wraparound time, reporting time relative to the transmission of a particular base station could incur other problems if the relationship of base station time to some global (e.g. Universal Time Coordinated (UTC) or Global Positioning System (GPS)) time was not known. Accordingly, it would be desirable to approximate a current global time, and to include the approximated current global time in an uplink message, thereby disambiguating the timing of uplink transmissions. It is noted that the global time may not in fact be a reference to any particular external timeline, as long as it is internally consistent.